1. Field of the Invention
The present invention relates to an optical submarine transmission system.
2. Description of the Related Art
FIG. 9 is a schematic of a conventional optical submarine transmission system. As shown in FIG. 9, in the optical submarine transmission system, cables 5, 6 laid in a land portion between land terminal stations 1 and 2 and beach manholes 3 and 4 are connected to a cable 7 laid in an underwater portion at the beach manholes 3 and 4. Black points in the beach manholes 3 and 4 represent connection points of the cables. A relay 8 is provided in the middle of the cable 7 in the underwater portion. A fiber that transmits signals from a land terminal station A1 to a land terminal station B2 and a fiber that transmits signals from the land terminal station B2 to the land terminal station A1 are depicted with a single cable.
In the conventional optical submarine transmission system, a backup line is laid in addition to a line that is usually used (hereinafter, “main line”). Therefore, the conventional system does not have a redundancy to be able to switch to the backup line when a fault occurs in the main line. It is preferable to configure the line to be redundant so that communication of signals can be performed without delay even when a fault occurs in the main line.
Recently, an output power of land terminal stations has improved according to improvement of an optical amplifier technology. Therefore, a long distance communication has become possible, as a result, the distance of land portions are lengthened. However, to lengthen the line of the land portion, it becomes more likely to cause faults on cables in the land portion due to external factors such as a construction. Also in this regard, redundancy of lines is desired.
Japanese Patent Application Laid-Open Publication No. 1993-206893 discloses an automatic switching method. In this method, a backup optical transmission path constituted of an optical transmission unit, an optical receiving unit, and an optical fiber connecting those two units is provided only in one downlink line from a control station to a base station. When an optical transmission path in any one of sectors has a fault, the optical transmission path is automatically switched to the backup optical transmission path. Thus, the optical transmission path is secured. A method of remedying a double fault in an optical transmission system is disclosed in Japanese Patent Application Laid-Open Publication No. 1993-316043. The optical transmission system includes a first optical switching unit that executes selection of a currently used optical transmitting unit and a backup optical transmitting unit and selection of a currently used optical transmitting unit and a backup optical receiving unit; a second optical switching unit connected to the first optical switching unit on one end and with a first to a fourth optical transmission paths on the other end through an optical dividing unit, and adapted to switch the optical transmission paths such that optical signals are transmitted and received using normally the first and the second optical transmission path, using, when any one of the first and the second optical transmission path is disconnected, the other optical transmission path and the third optical transmission path and using, when any one of the first and the second optical transmission path and the third optical transmission path are disconnected, the other optical transmission path and the fourth optical transmission path; and the optical dividing unit. In an optical transmission method disclosed in Japanese Patent Application Laid-Open Publication No. 1995-336296, each of subscriber apparatuses includes a transmission path fault monitoring unit for each of a currently-used-system and a backup-system, and a subscriber-apparatus star coupler. A fault is detected by receiving optical signals each having a different wavelength for each of the currently-used-system path and the backup-system transmission path.
A method of making a line redundant in an optical submarine transmission system is shown in FIG. 10. FIG. 10 is a schematic of the optical submarine transmission system that is fully duplex. As shown in FIG. 10, it can be considered that cables 9, 10, and 11 of a backup line (hereinafter, “backup line cable”) in addition to cables 5, 6, and 7 of a main line (hereinafter, “main line cable”) are laid in not only land portions but also in an underwater portion. Thus, the entire line between the land terminal station A1 and the land terminal station B2 is made duplex. However, the construction for the cable 11 and the relay 12 to be submerged requires tremendous costs. Therefore, it can be considered that only the line in the land portion is made duplex by laying a main line cable and a backup line cable between a beach manhole and a land terminal station. In this case, an optical switch for switching the lines is necessary.
However, space inside a beach manhole is limited and no commercial electric power is supplied to beach manholes. Therefore, active parts, such as an optical switch, cannot be installed in the beach manhole. Therefore, a line in a land portion between a beach manhole and a land terminal station can not be configured to be duplex. As shown in FIG. 10, for each of the main line and the backup line, a fiber for transmitting signals from the land terminal station A1 to the land terminal station B2 and a fiber for transmitting signals from the land terminal station B2 to the land terminal station A1 are combined to be depicted as one cable. In other words, the above conventional methods do not consider restrictions specific to an optical submarine transmission system.